The present invention relates to refractory articles that are used in the casting of steel, and particularly to such articles that are resistant to deposition onto the article""s walls of inclusions such as, for example, alumina and titania.
In the continuous casting of steel, refractory articles permit the transfer of molten steel between various containers, notably between the ladle and the distributor, and the distributor and the continuous casting mold. Such articles include, but are not limited to, stopper rods, shrouds, nozzles, and slide gate plates.
Refractory articles direct the flow of molten steel but also protect the steel from oxygen, which can reduce the quality of the steel. Despite these precautions, considerable amounts of oxygen can still dissolve in and react with the molten metal. Dissolved oxygen can precipitate from the steel and react with carbon to produce carbon monoxide. These gases create undesirable porosity, cracks and internal defects, which lower the quality of the finished steel. To eliminate dissolved oxygen, molten steel is often xe2x80x9ckilled,xe2x80x9d for example, by the addition of aluminum metal. In aluminum-killed steels, aluminum metal reacts with dissolved oxygen or iron oxide to form finely dispersed alumina, some of which floats into the slag above the molten metal and some of which remains as dispersed particles in the molten metal and the solidified steel.
These finely dispersed alumina particles have an affinity for carbon-bonded refractory materials, especially those containing graphite, which are commonly used in the continuous casting of steel. During casting, finely dispersed alumina may precipitate out of the molten metal onto the refractory surfaces. Alternatively, alumina may chemically react with and stick to the refractory surfaces. Accumulation of alumina on a nose of a stopper rod can prevent positive shut-off of the molten metal stream. Deposition in the casting channel of a nozzle, shroud or slide gate plate can clog the casting channel and substantially reduce the flow of molten steel.
Articles may be unclogged using an oxygen lance; however, lancing disrupts the casting process, reduces refractory life, and decreases casting efficiency and the quality of the steel produced. A total blockage of the casting channel by alumina decreases the expected life of the article and is very costly and time-consuming to steel producers. For example, steel having an initially high dissolved oxygen content can limit a shroud to 2-3 ladles due to heavy alumina buildup in the casting channel.
A common industrial technique to reduce alumina deposition is the injection of an inert gas, such as argon. The inert gas forms a protective barrier, and inhibits the finely dispersed alumina from precipitation on and reaction with graphite-containing refractories. The inert gas also reduces the partial pressure of oxygen around the molten metal farther decreasing formation and deposition of alumina. Exemplary of inert gas injection is UK patent application GB 2,111,880 A to Gruner et al. and U.S. Pat. No. 4,836,508 to Fishler, which describe a pour tube having a gas permeable refractory surrounding the casting channel. Unfortunately, gas injection requires large volumes of inert gas, complicated refractory designs, and is not always an effective solution. Inert gas at high pressure may also dissolve into the molten metal causing pinhole defects in the steel. Instead of, or in combination with, inert gas injection, a second refractory composition may be placed on refractory surfaces that are exposed to a stream of molten metal. For example, a surface composition may cover the nose of a stopper rod or the casting channel of a pour tube. The surface composition may be a lower melting point refractory, which sloughs off as alumina deposits on the surface. Such compositions include CaOxe2x80x94MgOxe2x80x94Al2O3 eutectics, as described in UK patent application GB 2,170,131 to Tate, or MgO according to UK patent application GB 2,135,918 to Rosenstock. These compositions tend to hydrate and are used up during casting. Their high thermal expansion coefficients can also cause surface cracking. For these reasons, the useful life of the surface layer is limited. To extend the life of the layer, refractory articles having compositions containing calcium oxide or calcium zirconate have been used. These compositions attempt to continuously replace CaO eutectics on the surface. Unfortunately, CaO does not diffuse to the surface quickly enough to be completely effective.
Other surface compositions, which inhibit alumina deposition, include SiAlON-graphite refractories as taught in U.S. Pat. No. 4,870,037 to Hoggard et al. and U.S. Pat. No. 4,871,698 to Fishler et al. SiAlON comprises a solid solution and/or dispersion of aluminum oxide and aluminum nitride in a silicon nitride matrix, and is believed to reduce wetting by molten metal. Graphite has superior thermal shock-resistance, and is often a major component in stopper rods, nozzles, shrouds and slide gate plates. Despite these benefits, SiAlON-graphite refractories are expensive, and graphite makes the composition susceptible to oxidation. Oxidation of the graphite accelerates alumina deposition and erosion of the refractory. To reduce oxidation, U.S. Pat. No. 5,185,300 to Hoggard et al. teaches metal diborides as sacrificial oxygen getters.
U.S. Pat. No. 5,691,061 to Hanse et al. teaches carbon-free surface compositions produced by the controlled oxidation of a carbon-containing material. The patent claims an initial composition of a metal oxide, carbon and a sintering precursor, and describes heating the composition, preferably above 1000xc2x0 C., in an oxidizing atmosphere leaving a densified, carbon-free, gas-impermeable material, which is resistant to alumina deposition. In practice, oxidation of the carbon is typically performed during preheating. Preheating is a common technique to raise the temperature of a refractory article before actual use, thereby reducing thermal shock to the article when contacting molten metal. Although eliminating problems associated with carbon oxidation, the preheating regime necessary to burn off the carbon and effect the required compositional changes is not always practical.
U.S. Pat. No. 5,286,685 to Schoennahl et al. describes a refractory composition, comprising a high melting point refractory, such as alumina, magnesia or MgOxe2x80x94Al2O3 spinel, aluminum nitride (AlN), and boron nitride. AlN is the bonding phase and, therefore, is said to avoid problems associated with carbon oxidation in carbon-bonded refractories. AlN-bonded refractories are purportedly resistant to alumina deposition, oxidation, erosion, do not promote reactions associated with alumina deposition, are resistant to thermal shock, and are not readily wetted by molten steel. AlN bonding occurs by shaping a piece comprising powdered aluminum metal and firing the piece in situ under a nitrogen atmosphere. This process is both dangerous, due to the presence of a reactive metal powder, expensive, and time consuming.
A need persists for an inexpensive, easily fabricated refractory composition that inhibits alumina deposition while resisting oxidation and erosion. Such a composition would be especially useful as a surface layer exposed to a stream of molten metal, for example, a stopper rod nose or a liner in the casting channel of a refractory nozzle, pour tube or slide gate plate.
The present invention describes a refractory article for use in the casting of molten steel that reduces the accumulation of inclusions, particularly alumina, on surfaces exposed to a stream of molten steel. The surface may be, for example, a nose of a stopper rod or a liner in a shroud, nozzle or slide gate plate.
In a broad aspect, the article comprises a first refractory composition forming the bulk of the article and a second refractory composition defining the contact surface. The first composition may be any number of refractory materials, such as carbon-bonded, oxide-bonded and castable materials. The second refractory composition is formed from a mixture including a refractory aggregate, a binder and a reactive metal. The mixture is cured below about 200xc2x0 C. to form a resin-bonded composition. After curing, the resin-bonded composition is preferably heat-treated below about 800xc2x0 C.
The second refractory composition is described as a cured, resin-bonded material in contrast to materials that are fired, carbon-bonded, oxide-bonded or comprise refractory cement. In one embodiment, the second composition comprises 50-90 wt. % refractory aggregate, 1-10 wt. % binder, and 0.5-15 wt. % reactive metal. A second composition may also include carbon, carbides and boron compounds. In another embodiment, the cured article includes 65-80 wt. % fused alumina, 2-30 wt. % calcined alumina, 0.5-10 wt. % aluminum metal, up to 15 wt. % zirconia and less than 1 wt. % silica. As a low temperature-treated material, the second refractory composition retains the reactive metal in a substantially unreacted state before preheating or casting operations.
Another aspect of the invention describes the first refractory composition as a fired material that is copressed with the second refractory composition. Another aspect teaches a first refractory composition that is cast around a pressed piece comprising the second refractory material.